1. FIELD OF THE INVENTION
This invention relates to a method of manufacturing semiconductor devices wherein a semiconductor device of a low forward power loss at a high current density and of a high switching speed characteristic is readily produced.
2. DESCRIPTION OF THE PRIOR ART
Conventionally, as a low operating voltage and high switching speed semiconductor device, a semiconductor device has been known which comprises a semiconductor wafer deposited with a metallic thin film forming a so-called Schottky barrier. For the deposition of the metallic thin film, electroplating, evaporation and the like have been used. With these measures, however, it was impossible to completely remove contaminations such as oxides created on the surface of the semiconductor wafer. With another method, such as sputtering, on account of slow deposition speed the semiconductor wafer suffered from contamination, heat or bombardment by charged particles during the sputtering, thereby resulting in deterioration of characteristics of the semiconductor devices. The deposition of the metallic thin film is also accomplished through a chemical vapor deposition method. With the last method, however, high deposition temperature deteriorated the semiconductor wafer and halogen gases gave rise to damage to the oxide film, thus preventing the production of desirable semiconductor devices. To conclude, the conventional methods as mentioned above succeeded in obtaining a semiconductor wafer in contact with a metallic film in a micro-area, but failed to produce large current capacity semiconductor devices through industrially available methods.
A recent development has been the ion implantation method in which an ion beam bombards the semiconductor wafer with the intention of forming a diffusion layer of a given concentration on a local area of the semiconductor wafer. With the ion implantation method, however, since an ion beam of high energy level is bombarded into a semiconductor substrate, a highly concentrated diffusion layer is formed in the semiconductor substrate in a thickness of more than 1000 A with the resultant creation of a P-N junction at the boundary between the diffusion layer and the semiconductor substrate. This inevitably results in a large forward voltage drop. In addition, it is impossible for the ion implantation method to form an ohmic contact on the surface of the diffusion layer, and there arises a need for providing the ohmic contact through other methods, for example, plating or evaporation.
A further proposal has been directed to high switching speed semiconductor devices manufactured by diffusing gold. In this proposal, a P-N junction is formed in a silicon semiconductor wafer, gold is diffused, and thereafter an ohmic contact is provided for an electrode. A silicon semiconductor device obtained therethrough reduces by the aid of diffused gold the lifetime of minority carriers in the silicon semiconductor wafer so that the number of residual minority carriers is decreased under the application of reverse voltage, thereby to suppress the effect of the minority carriers as much as possible. Accordingly, a high switching speed is obtainable, but the number of minority carriers is decreased and in addition, the P-N junction having inherently a large diffusion potential by itself, causes a large forward voltage drop.